The invention relates to new organopolysiloxanes containing phosphonic acid groups which can act as acid catalysts, cation exchangers and possess in their characteristics a number of advantages over organic polymer systems and inorganic supporting materials. In addition the metal salts of the organopolysiloxane phosphonic acids can be used to catalyse a wide variety of chemical transformations. Precursors of these new products, processes for their production and their uses are also described.
As is known, acid catalysts are utilised in the chemical and biochemical industry to conduct a wide range of chemical transformations. A range of homogenous and heterogeneous catalysts are used some of which require high temperatures to be effective and some produce considerable amount of bi-products and waste. These unwanted products and waste have to be treated and destroyed. The drive for more environmentally friendly processes—Green Chemistry—highlights the need for reusable, more effective and selective catalysts. This need has led to investigations into the design of new materials which can catalyse a variety of chemical transformations. Key requirements for such new catalysts are very good thermal stability, high insensitivity to chemical attack, high functional group loading, fixed and rigid structures, optimum functional groups so as to avoid rearrangements and side products, limited swelling capability, insolubility in organic solvents, ease of purification and high reusability, high ageing resistance and ease of access to the functional group which conducts the chemical transformation. In addition the processes to make such catalyst systems have to be flexible so as to enable the production of optimum structures and shapes for specific reactions. This could include tailoring the porosity from anywhere between macroporous to microporous structures, variable loading of the functional group, ease of making different metal derivatives and selective pH ranges.
A range of metals and catalysts have been embedded within or adsorbed on to the surface of silica, and other materials. The state of this art, for silica and its derivatives, is described by M. A. Brook in Silicon in Organic, Organometallic and Polymer Chemistry, Chapter 10, page 318, John Wiley & Sons, Inc., 2000. One of the problems encountered with these systems is the loss of the active functional groups due to their often very weak attachment to the silica New organo-silica materials are needed which whilst possessing the properties described above have functional groups which are strongly attached and which bind strongly to a range of metals and catalysts.
As a consequence of stricter environmental regulations there is a growing requirement for more effective systems for the removal and recovery of metals from a wide spectrum of metal contaminated solvents and aqueous based wastes and from contaminated waters. For example industries such as the nuclear industry and the electroplating industry generate substantial quantities of water-based effluent which are heavily contaminated with undesirable metal ions. Cation exchangers have been used to remove metal ions from solution and the state of this art is described in Kirk—Othmer's Encyclopaedia of Chemical Technology, 4th Edition, Vol. 14, page 737. The type of cation exchangers which are employed consist primarily of an organic, partly cross-linked polystyrene backbone with sulfonate groups attached to some of the phenyl rings. The physical and chemical properties of these polystyrene sulfonic cation exchangers are strongly affected by the organic nature of the polymeric backbone so that a number of disadvantages affect their technical field of application. The chemical and physical properties of a variety of such polystyrene based systems are described in the Bio-Rad Life Science Research Products catalogue 1998/99, pages 56–64. These limitations include relatively low temperature resistance 100°–130° C., sensitivity to chemical attack which can result in complete breakdown of the polymer matrix, strong swelling capacity, non-usability in certain organic solvents and the need for swelling to make the functional groups accessible. Organophosphonic acid cation exchangers have also been reported in U.S. Pat. No. 5,281,631. These systems are based on the products from the copolymerisation of the very expensive vinylidene disphosphonic acid with styrene, acrylonitrile and divinylbenzene. However the physical and chemical properties of these organophosphonic acid resins are very similar to the polystyrene sulfonic acid based systems and thus likewise their field of application is limited.
Inorganic polymer systems such as silica, aluminium oxide and titanium oxide, which do not suffer some of these drawbacks, have been investigated as ion exchangers. Active functional groups or metals are attached by a variety of means to these systems. However these systems suffer from the fact that only a low level of functional groups can be bound onto these surfaces. One of the additional problems encountered with these systems is that the functional groups can be removed on use or on standing. This is due to the rather weak attachment between the functional group and the surface atoms on the support.
Strong acidic cation exchangers based on sulfonic acid groups attached to a organopolysiloxane backbone have been described in U.S. Pat. No. 4,552,700 and U.S. Pat. No. 5,354,831. The materials reported have a general formula of (O3/2Si—R1—SO3−)xMx where R1 is an alkyl or cycloalkyl fragment, M is hydrogen or a mono to tetravalent metal ion and where the free valences of the oxygen atoms being saturated by silicon atoms of other groups of this formula and/or by cross-linking bridge members such as SiO4/2, R1SiO3/2, TiO4/2, AlO3/2, etc. Whilst these materials can act as cation exchangers it is generally recognised that sulfonic acid groups are limited in their effectiveness to complex with a range of metals and in comparison to other functional groups. In addition the sulfonate group is also limited by the fact that it is a mono anion and thus more of these functional groups are needed to bind to metals compared to other functional groups.